Athletes would like the sports authorities to find an analytical test that could guarantee that a particular supplement is free from any substances that could give rise to a positive urine test. This is not possible for a number of reasons:

1. The WADA list of prohibited substances is not definitive. By necessity the list identifies classes of Prohibited Substances, such as anabolic agents and related substances and Prohibited Methods only, rather than publishing a full list of substances. When analysing a supplement product for contaminants the Prohibited List is unable to act as a check list for all Prohibited Substances against which products should be screened - supplement analysis cannot guard against contamination using untried, untested and unknown designer drugs.
2. The sampling process for supplement testing is inherently inadequate as there will always be a large part of the product that is not analysed and it may be this part of the sample that contains the contaminant. This is particularly relevant for solid products such as powders, capsules, tablets and bars where the mixing of raw materials could potentially be variable (meaning that the contaminant may not be spread homogeneously throughout the whole product). More effective mixing is possible in the case of liquids, which means that the sample analysed is most likely to be representative of the whole product.

However, by screening a portion of a batch of product for the presence of banned substances using a highy sensitive and specialised test, supplement manufacturers can be alerted at an early stage to any possible trace contamination in the batch, allowing them to identify and eliminate the source. By carrying out this additional quality control measure the manufacturer can provide some assurance to an athlete that they would not be taking an undue risk when using their product.